LANs do not normally operate in isolation. They are connected to one another or to the Internet. To connect LANs, or segments ofLANs, we use connecting devices. Connecting devices can operate in different layers of the Internet model. In this chapter, we discuss only those that operate in the physical and data link layers.
Chapter 15 Connecting LANs, Backbone Networks, and Virtual LANs 15.1 Copyright © The McGrawHill Companies, Inc. Permission required for reproduction or display 15-1 CONNECTING DEVICES In this section, we divide connecting devices into five different categories based on the layer in which they operate in a network Topics discussed in this section: Passive Hubs Active Hubs Bridges TwoLayer Switches Routers ThreeLayer Switches Gateways 15.2 Figure 15.1 Five categories of connecting devices 15.3 Figure 15.2 A repeater connecting two segments of a LAN 15.4 Note A repeater connects segments of a LAN 15.5 Note A repeater forwards every frame; it has no filtering capability 15.6 Note A repeater is a regenerator, not an amplifier 15.7 Figure 15.3 Function of a repeater 15.8 Figure 15.4 A hierarchy of hubs 15.9 Note A bridge has a table used in filtering decisions 15.10 Figure 15.9 Finding the shortest paths and the spanning tree in a system of bridges 15.16 Figure 15.10 Forwarding and blocking ports after using spanning tree algorithm 15.17 Figure 15.11 Routers connecting independent LANs and WANs 15.18 15-2 BACKBONE NETWORKS A backbone network allows several LANs to be connected. In a backbone network, no station is directly connected to the backbone; the stations are part of a LAN, and the backbone connects the LANs. Topics discussed in this section: Bus Backbone Star Backbone Connecting Remote LANs 15.19 Note In a bus backbone, the topology of the backbone is a bus 15.20 Figure 15.12 Bus backbone 15.21 Note In a star backbone, the topology of the backbone is a star; the backbone is just one switch 15.22 Figure 15.13 Star backbone 15.23 Figure 15.14 Connecting remote LANs with bridges 15.24 Note A point-to-point link acts as a LAN in a remote backbone connected by remote bridges 15.25 15-3 VIRTUAL LANs We can roughly define a virtual local area network (VLAN) as a local area network configured by software, not by physical wiring Topics discussed in this section: Membership Configuration Communication between Switches IEEE Standard Advantages 15.26 Figure 15.15 A switch connecting three LANs 15.27 Figure 15.16 A switch using VLAN software 15.28 Figure 15.17 Two switches in a backbone using VLAN software 15.29 Note VLANs create broadcast domains 15.30 ... tree in a system of bridges 15. 16 Figure? ?15. 10 Forwarding? ?and? ?blocking ports after using spanning tree algorithm 15. 17 Figure? ?15. 11 Routers connecting independent LANs? ?and? ?WANs 15. 18 1 5- 2... IEEE Standard Advantages 15. 26 Figure? ?15. 15 A switch connecting three LANs 15. 27 Figure? ?15. 16 A switch using VLAN software 15. 28 Figure? ?15. 17 Two switches in a backbone using VLAN software 15. 29... Figure? ?15. 6 A learning bridge? ?and? ?the process of learning 15. 13 Figure? ?15. 7 Loop problem in a learning bridge 15. 14 Figure? ?15. 8 A system of connected LANs? ?and? ?its graph representation 15. 15 Figure? ?15. 9 Finding the shortest paths? ?and? ?the spanning